1. Field of the Invention
This invention relates to the field of high frequency magnetostatic surface wave delay lines and, more particularly, to a multiple magnetic layer composite useful in nondispersive and linearly dispersive magnetostatic surface wave delay lines.
2. Description of the Prior Art
At X-band frequencies and above, magnetic devices employing magnetostatic surface wave propagation have higher efficiency (lower losses) than can be achieved using either electromagnetic wave propagation in coaxial cable, bulk acoustic wave propagation, or surface acoustic wave propagation.
The time-delay versus frequency characteristic of a magnetostatic surface wave device employing only a single layer or film of magnetic-wave-active material is called monotonically dispersive. That is, the characteristic is neither nondispersive nor linearly dispersive over a sufficiently large band of frequencies to be of practical interest.
The prior art reveals several approaches to providing a nondispersive or linearly dispersive characteristic for a magnetostatic surface wave delay line. Disposing a layer of a magnetic-wave-propagation-modifying material such as an electrically conductive ground plane in the vicinity of a layer of magnetic-wave-active material significantly alters the time-delay versus frequency characteristic. The altered characteristic approximates nondispersive or linearly dispersive behavior in the vicinity of points of inflection in the characteristic. The points of inflection appear when the ground plane is used. The approximation is improved and the bandwidth of a region of nondispersive or linearly dispersive behavior is broadened by using more than one layer of magnetically-coupled, magnetic-wave-active material in the delay line. When the layers of magnetic-wave-active material in a layered composite have different magnetizations, their points of inflection appear in the overall characteristic displaced from each other. This produces the bandwidth broadening effect in the overall time-delay characteristic. The magnetization of each of the plurality of layers of magnetic-wave-active material can be due to differences in the internal magnetizations of different materials, differences in the crystallographic orientation of the layers of material relative to the bias magnetic field, spatial variation or non-uniformity in the bias magnetic field, or a combination of some or all of these factors.
3. Prior Art Statement
The most pertinent prior art discovered by applicant relative to this invention is listed herewith:
(1) U.S. Pat. No. 3,864,647, Bongianni, "Substantially Linear Magnetic Dispersive Delay Line and Method of Operating It" (1975). PA1 (2) The Microwave Journal, (Bongianni), January 1974, pp. 49-52, 71, "X-Band Signal Processing Using Magnetic Waves". PA1 (3) Journal of Applied Physics, (Ganguly et al.), Vol. 45, No. 10, October 1974, pp. 4665-4667, "Magnetostatic Wave Propagation in double layers of magnetically anisotropic slabs". PA1 (4) IEEE Transactions on Microwave Theory and Techniques, (Tsutsumi et al.), Vol. MTT-24, No. 9, September 1976, pp. 591-597, "Effect of the Magnetic Perturbation on Magnetostatic Surface-Wave Propagation".
The patent to Bongianni, discloses a layered composite arrangement in a magnetostatic surface wave device wherein a plurality of layers of magnetic-wave-active material are spaced apart from each other and from a ground plane by intervening layers of magnetic-wave-inactive materials. The layers of magnetic-wave-active material have similar, preferably substantially identical, characteristics. The similarity prevents the response of a more efficient layer from dominating the response of a lossier layer. A preferred embodiment is disclosed which comprises films of yttrium iron garnet (YIG) deposited on both sides of the gadolinium gallium garnet (GGG) substrate in a liquid phase epitaxy (LPE) system. Each magnetic-wave-active layer is given a magnetic characteristic different from other layers by causing the bias magnetic field for the delay line to have a substantial non-zero gradient in a direction perpendicular to the layers. A three-magnet configuration is disclosed for producing such a field.
The article by Bongianni discloses substantially the same device as the first listed reference.
In the article by Ganguly et al., a layered composite for a magnetostatic surface wave device is disclosed wherein substantially similar but physically distinct slabs of magnetic-wave-active material are given a different crystallographic orientation relative to a uniform bias magnetic field. The different orientation relative to the field causes each slab to have different magnetic characteristics.
The article by Tsutsumi et al, discloses a layered composite for a magnetostatic surface wave device wherein two physically distinct slabs of different magnetic-wave-active materials (namely, YIG and Ga-YIG), each having different magnetic saturations, are stacked in a uniform bias magnetic field. The difference in the internal magnetization of the two different materials provides the differing magnetic characteristics.
None of the above-listed references disclose a monolithic multiple magnetic layer composite for use in a nondispersive or linearly dispersive magnetostatic surface wave delay line having a uniform bias magnetic field wherein two films having different magnetic characteristics are epitaxially deposited on opposite sides of the same magnetic-wave-inactive substrate.